Lubricating oil composition for high-temperature use

ABSTRACT

The present invention relates to a lubricant composition for a high temperature of which the amount evaporated in a thin film at a high temperature is controlled and of which the fluidity is maintained for a long period of time, and it is a lubricant composition for a high temperature which is obtained by mixing a base oil composition comprising (a) 20 to 100% by weight of a specific aromatic ester and (b) 0 to 80% by weight of another base oil with (c) 0.1 to 10% by weight, based on the total amount of the lubricant composition, of an antioxidant.

TECHNICAL FIELD

The present invention relates to a lubricant composition for a hightemperature. More specifically, it relates to a lubricant compositionfor a high temperature which is used in a chain, a roller chain, a chainconveyor, a bearing and the like.

BACKGROUND OF THE INVENTION

Since a lubricant which is used ina chain, a roller chain, a chainconveyor, a bearing and the like is exposed to a high temperature, anamount of a lubricant evaporated greatly influences a life of anapparatus. Thus, a lubricant loses an ordinary viscosity under acondition of a high temperature and becomes a thin film, so that anamount of a lubricant evaporated has to be controlled under severerconditions. As an ordinary lubricant for a high temperature,high-molecular oil with a high viscosity has been used to control anamount evaporated. Although an amount of such oil evaporated is small, apower loss is great, and it is undesirable in view of a totalperformance of a lubricant. Further, when such oil is exposed to a thinfilm and a high temperature, a residual amount is large, but it issolidified. Not only are characteristics as a liquid lost, but also itbecomes a solidified sludge to prevent flow of oil, inviting poorlubrication of a lubricating portion. Accordingly, a lubricant for ahigh temperature of which the amount evaporated in a thin film at a hightemperature is controlled and of which the fluidity is maintained for along period of time has been in demand.

From this standpoint, the present invention has been made, and it aimsto provide a lubricant composition for a high temperature of which theamount evaporated in a thin film at a high temperature is controlled andof which the fluidity is maintained for a long period of time.

DISCLOSURE OF THE INVENTION

The present inventors have assiduously conducted investigations, andhave consequently found that the aim of the present invention can beachieved by using a specific aromatic ester compound as base oil. Thisfinding has led to the completion of the present invention.

That is, the gist of the present invention is as follows.

-   -   (1) A lubricant composition for a high temperature which is        obtained by mixing a base oil composition comprising (a) 20 to        100% by weight of an aromatic ester compound represented by the        following general formula (I)        -   (wherein R¹ represents an alkyl group having 6 to 16 carbon            atoms, and n represents an integer of 1 to 6)    -    and (b) 0 to 80% by weight of another base oil with (c) 0.1 to        10% by weight, based on the total amount of the lubricant        composition, of an antioxidant.    -   (2) The lubricant composition for the high temperature as        recited in the above (1), wherein the base oil composition        comprises (a) 30 to 80% by weight of the aromatic ester compound        and (b) 20 to 70% by weight of another base oil.    -   (3) The lubricant composition for the high temperature as        recited in the above (1) or (2), wherein the aromatic ester        compound (a) is one selected from a trimellitic acid alkyl ester        and a pyromellitic acid alkyl ester.    -   (4) The lubricant composition for the high temperature as        recited in any of the above (1) to (3), wherein another base        oil (b) is one type or two or more types selected from an        α-olefin oligomer, an ethylene-α-olefin oligomer, polybutene and        hydrogenated substances thereof.    -   (5) The lubricant composition for the high temperature as        recited in any of the above (1) to (4), wherein the        antioxidant (c) is one containing sulfur and/or phosphorus in a        molecule.    -   (6) The lubricant composition for the high temperature as        recited in any of the above (1) to (5), wherein the antioxidant        is one selected from a dithiophosphoric acid salt and a        thiophosphoric acid ester.    -   (7) The lubricant composition for the high temperature as        recited in any of the above (1) to (5), wherein the antioxidant        is        2,6-di-tert-butyl-4-(4,6-bis(octylthio)-1,3,5-triazin-2-ylamino)phenol.

BEST MODE FOR CARRYING OUT THE INVENTION

The mode for carrying out the present invention is described below.

The base oil composition constituting the lubricant composition for thehigh temperature in the present invention comprises (a) 20 to 100% byweight of the aromatic ester compound represented by the above generalformula (I) and (b) 0 to 80% by weight of another base oil.

With respect to the aromatic carboxylic acid constituting the aromaticester compound as component (a), the number of carboxylic acids bound tobenzene is preferably 3 or 4. Among others, trimellitic acid andpyromellitic acid are preferable. As an aliphatic alcohol used in thearomatic carboxylic acid ester, a linear or branched alcohol having 6 to16 carbon atoms is preferable. Specific examples thereof can includetri-n-hexyl trimellitate, tri-2-ethylhexyl trimellitate, tri-n-octyltrimellitate, tri-3,5,5-trimethylhexyl trimellitate, tetra-n-hexylpyromellitate, tetra-2-ethylhexyl pyromellitate, tetra-n-octylpyromellitate, tetra-3,5,5-trimethylhexyl pyromellitate and the like.Incidentally, the aromatic carboxylic acid esters may be used eithersingly or in admixture of two or more types, and a partial ester may becontained in a full ester.

As another base oil (b), any mineral oil type or synthetic oil type thatis used as base oil of ordinary equipment oil can be used. As themineral oil-type base oil, for example, refined oil which is formed byrefining a lubricant fraction resulting from atmospheric distillation orvacuum distillation of paraffin base crude oil, intermediate base crudeoil or naphthene base crude oil by an ordinary method such as solventdeasphalting, solvent extraction, hydrocracking, solvent dewaxing,catalytic dewaxing, hydrogenation refining, sulfuric acid treatment,clay treatment or the like.

Further, as the synthetic oil-type base oil, various substances such asan α-olefin oligomer, an ethylene-α-oligomer, a polybutene, a dibasicacid ester, a polyalkylene glycol, a hindered ester, an alkylbenzene, analkylnaphthalene, a polyether and the like can be used. Of these, anα-olefin oligomer, an ethylene-α-oligomer, a polybutene or hydrogenatedsubstances thereof are preferable because the fluidity can be maintainedfor a long period of time. Incidentally, as component (b), one type or amixture of two or more types may be used, and a mixture of mineral oiland synthetic oil may be used.

The proportions of the aromatic carboxylic acid ester (a) and anotherbase oil (b) are that component (a) is 20 to 100% by weight andcomponent (b) is 0 to 80% by weight, and preferably component (a) is 30to 80% by weight and component (b) is 20 to 70% by weight. Whencomponent (a) is less than 20% by weight, the effect of the invention isnot provided.

Next, with respect to the antioxidant as component (c) mixed with baseoil, an amine-type antioxidant and a phenolic antioxidant can be used.In view of the effect, an antioxidant containing sulfur and/orphosphorus in a molecule is preferable.

Examples of the amine-type antioxidant include monoalkyldiphenylaminessuch as monooctyldiphenylamine, monononyldiphenylamine and the like;dialkyldiphenylamines such as 4,4′-dibutyldiphenylamine,4,4′-dipentyldiphenylamine, 4,4′-dihexyldiphenylamine,4,4′-diheptyldiphenylamine, 4,4′-dioctyldiphenylamine,4,4′-dinonyldiphenylamine and the like; polyalkyldiphenylamines such astetrabutyldiphenylamine, tetrahexyldiphenylamine,tetraoctyldiphenylamine, tetranonyldiphenylamine and the like; andnaphthylamines such as α-naphthylamine, phenyl-α-naphthylamine,butylphenyl-α-naphthylamine, pentylphenyl-α-naphthylamine,hexylphenyl-α-naphthylamine, heptylphenyl-α-naphthylamine,octylphenyl-α-naphthylamine, nonylphenyl-α-naphthylamine and the like.

Examples of the phenolic antioxidant can include monophenols such as2,6-di-tert-butyl-4-methylphenol, 2,6-di-tert-butyl-4-ethylphenol andthe like; and diphenols such as4,4′-methylenebis(2,6-di-tert-butylphenol),2,2′-methylenebis(4-ethyl-6-tert-butylphenol) and the like.

Examples of the antioxidant containing sulfur and/or phosphorus in themolecule can be an additive containing sulfur and/or phosphorus in amolecule and having an antioxidant ability, and it may be added for theother purpose.

First, the antioxidant containing sulfur in the molecule is described.For example, sulfurized oil, sulfurized mineral oil, a sulfurized fattyacid, a sulfurized ester, a sulfurized olefin, dihydrocarbylpolysulfide, a thiadiazole compound, an alkylthiocarbamoyl compound, atriazine compound, a thioterpene compound, a dialkylthiodipropionatecompound and the like can be mentioned.

Here, sulfurized oil is oil obtained by reacting sulfur or asulfur-containing compound with oil (lard, whale oil, vegetable oil,fish oil or the like), and the sulfur content is not particularlylimited. Generally, it is preferably between 5 and 30% by weight.Specific examples thereof include sulfurized lard, sulfurized rapeseedoil, sulfurized castor oil, sulfurized soybean oil, sulfurized rice branoil and the like. Examples of the sulfurized fatty acid includesulfurized oleic acid and the like. Examples of the sulfurized esterinclude sulfurized oleic acid methyl ester, sulfurized rice bran fattyacid octyl ester and the like.

As the sulfurized olefin, for example, a compound represented by thefollowing general formula (II)R²—S_(a)—R³  (II)

-   -   (wherein R² represents an alkenyl group having 2 to 15 carbon        atoms, R³ represents an alkyl group or an alkenyl group having 2        to 15 carbon atoms, and a represents an integer of 1 to 8).        This compound is obtained by reacting an olefin having 2 to 15        carbon atoms or its dimer to tetramer with a sulfurizing agent        such as sulfur, sulfur chloride or the like. As the olefin,        propylene, isobutene, diisobutene or the like is preferable.

Further, the dihydrocarbyl polysulfide is a compound represented by thefollowing general formula (III)R⁴—S_(b)—R⁵  (III)

-   -   (wherein R⁴ and R⁵ each represent an alkyl group or a cyclic        alkyl group having 1 to 20 carbon atoms, an aryl group having 6        to 20 carbon atoms, an alkylaryl group having 7 to 20 carbon        atoms or an arylalkyl group having 7 to 20 carbon atoms, and        these may be the same or different, and b represents an integer        of 2 to 8).        When R⁴ and Rs therein are alkyl groups, it is called an alkyl        sulfide.

Specific examples of R⁴ and R⁵ in the above general formula (III)include a methyl group, an ethyl group, an n-propyl group, an isopropylgroup, an n-butyl group, an isobutyl group, a sec-butyl group, atert-butyl group, various pentyl groups, various hexyl groups, variousheptyl groups, various octyl groups, various nonyl groups, various decylgroups, various dodecyl groups, a cyclohexyl group, a cyclooctyl group,a phenyl group, a naphthyl group, a tolyl group, a xylyl group, a benzylgroup, a phenetyl group and the like.

Preferable examples of the dihydrocarbyl polysulfide include dibenzylpolysulfide, various dinonyl polysulfides, various didodecylpolysulfides, various dibutyl polysulfides, various dioctylpolysulfides, diphenyl polysulfide, dicyclohexyl polysulfide and thelike.

As the thiadiazole compound, for example, 1,3,4-thiadiazole and1,2,4-thiadiazole compounds represented by the following generalformulas (IV)

-   -   (wherein R⁶ and R⁷ each represent a hydrogen atom or a        hydrocarbon group having 1 to 20 carbon atoms, and c and d each        represent an integer of 0 to 8)        are preferably used.

Specific examples of the thiadiazole compounds can include2,5-bis(n-hexyldithio)-1,3,4-thiadiazole,2,5-bis(n-octyldithio)-1,3,4-thiadiazole,2,5-bis(n-nonyldithio)-1,3,4-thiadiazole,2,5-bis(1,1,3,3-tetramethylbutyldithio)-1,3,4-thiadiazole,3,5-bis(n-hexyldithio)-1,2,4-thiadiazole,3,5-bis(n-octyldithio)-1,2,4-thiadiazole,3,5-bis(n-nonyldithio)-1,2,4-thiadiazole,3,5-bis(1,1,3,3-tetramethylbutyldithio)-1,2,4-thiadiazole and the like.

As the alkylthiocarbamoyl compound, for example, a compound representedby the following general formula (V)

-   -   (wherein R⁸ to R¹¹ each represent an alkyl group having 1 to 20        carbon atoms, and e represents an integer of 1 to 8)        can preferably be used.

Specific examples of the alkylthiocarbamoyl compound includebis(dimethylthiocarbamoyl) monosulfide, bis(dibutylthiocarbamoyl)monosulfide, bis(dimethylthiocarbamoyl) disulfide,bis(dibutylthiocarbamoyl) disulfide, bis(diamylthiocarbamoyl) disulfide,bis(dioctylthiocarbamoyl) disulfide and the like.

Preferable examples of the triazine compound include2,6-di-tert-butyl-4-(4,6-bis(octylthio)-1,3,5-triazin-2-ylamino)phenoland the like.

Further, examples of the thioterpene compound include a reaction productof phosphorus pentasulfide and pinene. Examples of thedialkylthiodipropionate compound include dilaurylthiodipropionate,distearylthiodipropionate and the like.

Further, as the antioxidant containing sulfur and a metal, a zincdialkyldithiocarbamate (Zn-DTC), a molybdenum dialkyldithiocarbamate(Mo-DTC), a lead dialkyldithiocarbamate, a tin dialkyldithiocarbamate,sodium sulfonate, calcium sulfonate and the like can also be used.

Next, the antioxidant containing phosphorus in the molecule isdescribed. Typical examples include phosphoric acid esters and aminesalts thereof.

The phosphoric acid esters include a phosphoric acid ester, an acidphosphoric acid ester, a phosphorous acid ester and an acid phosphorousacid ester represented by the following general formulas (VI) to (X).

In the general formulas (VI) to (X), R¹² to R¹⁴ each represent an alkylgroup, an alkenyl group, an alkylaryl group and an arylalkyl grouphaving 4 to 30 carbon atoms, and R¹² to R¹⁴ may be the same ordifferent.

The phosphoric acid ester includes a triaryl phosphate, a trialkylphosphate, a trialkylaryl phosphate, a triarylalkyl phosphate, atrialkenyl phosphate and the like. Specific examples thereof includetriphenyl phosphate, tricresyl phosphate, benzyldiphenyl phosphate,ethyldiphenyl phosphate, tributyl phosphate, ethyldibutyl phosphate,cresyldiphenyl phosphate, dicresylphenyl phosphate, ethylphenyldiphenylphosphate, diethylphenylphenyl phosphate, propylphenyldiphenylphosphate, dipropylphenylphenyl phosphate, triethylphenyl phosphate,tripropylphenyl phosphate, butylphenyldiphenyl phosphate,dibutylphenylphenyl phosphate, tributylphenyl phosphate, trihexylphosphate, tri(2-ethylhexyl) phosphate, tridecyl phosphate, trilaurylphosphate, trimyristyl phosphate, tripalmityl phosphate, tristearylphosphate, trioleyl phosphate and the like.

Specific examples of the acid phosphoric acid ester include 2-ethylhexylacid phosphate, ethyl acid phosphate, butyl acid phosphate, oleyl acidphosphate, tetracosyl acid phosphate, isodecyl acid phosphate, laurylacid phosphate, tridecyl acid phosphate, stearyl acid phosphate,isostearyl acid phosphate and the like.

Specific examples of the phosphorous acid ester include triethylphosphite, tributyl phosphite, triphenyl phosphite, tricresyl phosphite,tri(nonylphenyl) phosphite, tri(2-ethylhexyl) phosphite, tridecylphosphite, trilauryl phosphite, triisooctyl phosphite, diphenylisodecylphosphite, tristearyl phosphite, trioleyl phosphite and the like.

Specific examples of the acid phosphorous acid ester includedibutylhydrogen phosphite, dilaurylhydrogen phosphite, dioleylhydrogenphosphite, distearylhydrogen phosphite, diphenylhydrogen phosphite andthe like.

Further, the amines forming amine salts with these include amono-substituted amine, a di-substituted amine and a tri-substitutedamine represented by the general formula (XI)R¹⁵ _(m)NH_(3-m)  (XI)

-   -   (wherein R¹⁵ represents an alkyl group or an alkenyl group        having 3 to 30 carbon atoms, an aryl group or an arylalkyl group        having 6 to 30 carbon atoms or a hydroxyalkyl group having 2 to        30 carbon atoms, m represents 1, 2 or 3, and when R¹⁵ is plural        groups, plural R¹⁵'s may be the same or different).        In R¹⁵ in the general formula (XI), an alkyl group or an alkenyl        group having 3 to 30 carbon atoms may be linear, branched or        cyclic.

Here, examples of the mono-substituted amine can include butylamine,pentylamine, hexylamine, cyclohexylamine, octylamine, laurylamine,stearylamine, oleylamine, benzylamine and the like. Examples of thedi-substituted amine can include dibutylamine, dipentylamine,dihexylamine, dicyclohexylamine, dioctylamine, dilaurylamine,distearylamine, dioleylamine, dibenzylamine, stearyl-monoethanolamine,decyl-monoethanolamine, hexyl-monopropanolamine,benzyl-monoethanolamine, phenyl-monoethanolamine, tolyl-monopropanolamide and the like. Examples of the tri-substituted amine includetributylamine, tripentylamine, trihexylamine, tricyclohexylamine,trioctylamine, trilaurylamine, tristearylamine, trioleylamine,tribenzylamine, dioleyl-monoethanolamine, dilauryl monopropanolamine,dioctyl-monoethanolamine, dihexyl-monopropanolamine,dibutyl-monopropanolamine, oleyl-diethanol amine,stearyl-dipropanolamine, lauryl-diethanolamine, octyl-dipropanolamine,butyl-diethanolamine, benzyl-diethanolamine, phenyl-diethanolamine,tolyl-dipropanolamine, xylyl-diethanolamine, triethanolamine,tripropanolamine and the like.

Examples of the antioxidant containing phosphorus and a halogen atom caninclude a chlorinated phosphoric acid ester.

Examples of the antioxidant containing sulfur and phosphorus in themolecule can include phosphorus sulfide oil, a phosphorus sulfideolefin, a thiophosphoric acid ester (thiophosphite and thiophosphate)and the like. Of these, a thiophosphite (alkylaryl type) and athiophosphate (alkylaryl type) are preferable.

Examples of the antioxidant containing sulfur, phosphorus and a metalcan include dithiophosphoric acid salts such as zinc dithiophosphate(Zn-DTP), molybdenum dithiophosphate (Mo-DTP), lead dithiophosphate, tindithiophosphate and the like.

Among the above-described antioxidants,2,6-tert-butyl-4-(4,6-bis(octylthio)-1,3,5-triazin-2-ylamino)phenol,Zn-DTP(alkylaryl type), a thiophosphite (alkyl type) and a thiophosphate(alkylaryl type) are preferable.

Further, the above-described antioxidants can be used either singly orin combination.

The preferable mixing amount of the above antioxidant is in the range of0.1 to 10% by weight based on the total amount of the composition. Whenthe mixing amount is less than 0.1% by weight, the aim of the presentinvention is not fully exhibited. When it exceeds 10% by weight, theimprovement in the effect is sometimes not observed even with thisamount, and the solubility in base oil is sometimes decreased. The morepreferable mixing amount is in the range of 1 to 7% by weight.

The lubricant composition for the high temperature in the presentinvention can contain various known additives such as a rust-proofingagent, a detergent-dispersant, a metal inactivator, a defoamer and thelike as required unless the aim of the present invention is impaired.

Examples of the rust-proofing agent can include a metal sulfonate, asuccinic acid ester and the like.

Examples of the detergent-dispersant can include a metal sulfonate, ametal salicylate, a metal finate, a succinic acid imide and the like.

Examples of the metal inactivator can include benzotriazole, thiadiazoleand the like.

Examples of the defoamer include methyl silicone oil, fluorosiliconeoil, a polyacrylate and the like.

It is advisable that the kinematic viscosity at 40° C. of the lubricantcomposition for the high temperature in the present invention isadjusted to 10 to 50 mm²/s in order to achieve the aim. It is morepreferably in the range of 50 to 320 mm²/s.

Incidentally, the lubricant composition for the high temperature in thepresent invention is preferably used when the temperature exceeds 150°C. in a time of more than 50% of the use period.

Next, the present invention is illustrated more specifically withreference to Examples. However, the present invention is not limited tothese Examples at all.

Examples 1 to 6 and Comparative Examples 1 and 2

As shown in Table 1, base oil was mixed with an antioxidant to prepare alubricant composition. With respect to the viscosity of the composition,the mixing was conducted by adjusting a viscosity of an α-olefinoligomer, whereby the kinematic viscosity at 40° C. was all adjusted to220 mm²/s. Regarding the composition, a thin film residue test wasconducted in the following manner. Further, a container was inclined,and the fluidity at that time was visually evaluated. The results areshown in Table 1.

Thin film residue test

The amount of the residue was measured at 200° C. for 24 hours using acontainer and a constant-temperature air bath indicated ina lubricantheat stability test of JIS K2540. This was expressed in terms ofpercentage, and defined as a residue ratio. By the way, 10 liters/hr ofair was always caused to flow in during the measurement.

TABLE 1 Ex. 1 Ex. 2 Ex. 3 Ex. 4 (1) Mixing α-olefin oligomer^(*1)balance balance balance balance ratio polybutene^(*2) 20 20 20 20 wt. %aromatic ester 1^(*3) 50 50 50 50 aromatic ester 2^(*4) — — — —bisphenol^(*5) 1 1 1 1 diphenylamine^(*6) 1 1 1 1 dithiophosphoric acid— 2 — — salt^(*7) thiophosphoric acid — — 2 — ester^(*8) triazine-typesulfur- — — — 2 containing compound^(*9) Residue ratio (%) 62.5 70.267.3 76.6 Fluidity (visual observation) yes yes yes yes Ex. 5 Ex. 6 CEx.1 CEx. 2 (2) Mixing α-olefin oligomer^(*1) balance balance balancebalance ratio polybutene^(*2) 20 20 20 20 wt. % aromatic ester 1^(*3) —— — — aromatic ester 2^(*4) 50 50 — — bisphenol^(*5) 1 1 1 1diphenylamine^(*6) 1 1 1 1 dithiophosphoric acid — 2 — 2 salt^(*7)thiophosphoric acid — 2 — 2 ester^(*8) triazine-type sulfur- 2 — — —containing compound^(*9) Residue ratio (%) 78.1 84.1 50.6 55.2 Fluidity(visual observation) yes yes no no Ex.—Example, CEx.—Comparative Example(Notes) ^(*1)Hydrogenated substance of an oligomer, 1-decene (kinematicviscosity at 40° C. 30 to 63 mm²/s) ^(*2)Number average molecularweight: 2,300 ^(*3)tri-n-ocytl trimellitate ^(*4)tri-n-octylpyromellitate ^(*5)4,4′-methylenebis(2,6-di-tert-butylphenol)^(*6)4,4′dibutyldiphenylamine ^(*7)alkylaryl-type Zn-DTP^(*8)triisononylphenol thiophosphate^(*9)2,6-di-tert-butyl-4-(4,6-bis(octylthio)-1,3,5-triazin-2-ylamino)phenol

Industrial Applicability

According to the present invention, a lubricant composition for a hightemperature of which the amount evaporated in a thin film at a hightemperature is controlled and of which the fluidity is maintained for along period of time can be provided.

1. A lubricant composition for a high temperature which is obtained bymixing a base oil composition comprising (a) 20 to 80% by weight of anaromatic ester compound represented by the following general formula (I)

( wherein R¹ represents an alkyl group having 6 to 16 carbon atoms, andn represents an integer of 1 to 6) , and (b) 20 to 80% by weight ofanother base oil with (c) 0.1 to 10% by weight, based on the totalamount of the lubricant composition, of an antioxidant, wherein saidanother base oil includes an α-olefin oligomer, or hydrogenated productsthereof, or an ethylene-α-olefin oligomer, or hydrogenated productsthereof of said ethylene-α-olefin oligomer.
 2. The lubricant compositionfor the high temperature as claimed in claim 1, wherein the base oilcomposition comprises (a) 30 to 80% by weight of the aromatic estercompound and (b) 20 to 70% by weight of another base oil.
 3. Thelubricant composition for the high temperature as claimed in claim 1,wherein the aromatic ester compound (a) is one selected from atrimellitic acid alkyl ester and a pyromellitic acid alkyl ester.
 4. Thelubricant composition for the high temperature as claimed in claim 1,wherein said another base oil (b) also includes a polybutene orhydrogenated products thereof.
 5. The lubricant composition for the hightemperature as claimed in claim 1, wherein the antioxidant (c) is onecontaining sulfur and/or phosphorus in a molecule.
 6. The lubricantcomposition for the high temperature as claimed in claim 1, wherein theantioxidant (c) is one selected from a dithiophosphoric acid salt and athiophosphoric acid ester.
 7. The lubricant composition for the hightemperature as claimed in claim 1, wherein the antioxidant (c) is2,6-di-tert-butyl-4-(4,6-bis(octylthio)-1,3,5-triazin-2-ylamino)phenol.8. The lubricant composition for the high temperature as claimed inclaim 1, wherein n is 3 or
 4. 9. The lubricant composition for the hightemperature as claimed in claim 1, wherein n is
 3. 10. The lubricantcomposition for the high temperature as claimed in claim 1, wherein n is4.
 11. A lubricant composition for a high temperature which is obtainedby mixing a base oil composition comprising (a) 20 to 80 % by weight ofan aromatic ester compound represented by the following general formula(I)

wherein R ¹ represents an alkyl group having 6 to 16 carbon atoms, and nrepresents an integer of 3 to 6, and (b) 20 to 80 % by weight of anotherbase oil with (c) 0.1 to 10 % by weight, based on the total amount ofthe lubricant composition, of an antioxidant, wherein said another baseoil includes an α-olefin oligomer, or hydrogenated products thereof, oran ethylene-α-olefin oligomer, or hydrogenated products thereof.
 12. Thelubricant composition for the high temperature as claimed in claim 11,wherein the base oil composition comprises (a) 30 to 80 % by weight ofthe aromatic ester compound and (b) 20 to 70 % by weight of another baseoil.
 13. The lubricant composition for the high temperature as claimedin claim 11, wherein the aromatic ester compound (a) is one selectedfrom a trimellitic acid alkyl ester and a pyromellitic acid alkyl ester.14. The lubricant composition for the high temperature as claimed inclaim 11, wherein said another base oil (b) also includes a polybuteneor hydrogenated products thereof.
 15. The lubricant composition for thehigh temperature as claimed in claim 11, wherein the antioxidant (c) isone containing sulfur and/or phosphorus in a molecule.
 16. The lubricantcomposition for the high temperature as claimed in claim 11, wherein theantioxidant (c) is one selected from a dithiophosphoric acid salt and athiophosphoric acid ester.
 17. The lubricant composition for the hightemperature as claimed in claim 11, wherein the antioxidant (c) is 2,6-di-tert-butyl- 4 -( 4,6 -bis(octylthio)- 1,3,5 -triazin- 2-ylamino)phenol.
 18. The lubricant composition for the high temperatureas claimed in claim 11, wherein n is 3 or
 4. 19. The lubricantcomposition for the high temperature as claimed in claim 11, wherein nis
 3. 20. The lubricant composition for the high temperature as claimedin claim 11, wherein n is
 4. 21. A lubricant composition for a hightemperature which is obtained by mixing a base oil compositioncomprising (a) 20 to 80 % by weight of an aromatic ester compoundrepresented by the following general formula (I)

wherein R ¹ represents an alkyl group having 6 to 16 carbon atoms, and nrepresents an integer of 1 to 6, and (b) 20 to 80 % by weight of anotherbase oil with (c) 0.1 to 10 % by weight, based on the total amount ofthe lubricant composition, of a zinc dithiophosphoric acid saltantioxidant, wherein said another base oil includes an α-olefinoligomer, or hydrogenated products thereof, or an ethylene-α-olefinoligomer, or hydrogenated products thereof.